Steam-Driven Blasts Last Seen at K&imacr;lauea in 1924 May Recur

An explosion at Halema’uma’u crater on 18 May 1924. One person was killed by falling debris during this explosive sequence. Credit: U.S. Geological Survey; photo by Kenichi Maehara, courtesy of Bishop Museum

As a textbook example of an effusive volcano, Hawaii’s Kīlauea is known for liquid, oozing lava that erupts in flows, fountains, and splatters. But scientists at the Hawaiian Volcano Observatory (HVO) are now warning that conditions are right for rare explosive eruptions that could launch “ballistic blocks” the size of refrigerators in the summit area, and blanket downwind communities with a layer of ash.

The U.S. Geological Survey (USGS) issued the warning for explosive eruptions this past Wednesday, after the lava lake in Kīlauea’s summit crater, called Halema’uma’u (or “House of Ferns”), dropped to a level they believed was near the water table.

Ordinarily, magma in a volcano’s internal plumbing heats the surrounding rock, sealing it against water movement, explained Jessica Johnson, a volcano geophysicist at the University of East Anglia in the United Kingdom, who spent 2011–2013 on a research fellowship at Kīlauea. However, when the magma level falls below the water table and the rock above it cools, small cracks open up through which water can flow into the conduits, she noted. The water can become trapped underground as rocks falling down the sides of the emptying crater plug the vent.

“The pressure builds up, and that’s when you get a larger explosion.”When heat from the magma turns the water into steam, “the pressure builds up, and that’s when you get a larger explosion,” Johnson said.

Unusual for Kīlauea

If such explosions were to happen during the current effusive eruption episode, which began on 3 May, it would be the first time that explosive eruptions would occur at Kīlauea since 1924. Back then, scientists counted more than 50 massive explosions over a two-and-a-half-week period. The summit crater of Halema’uma’u doubled in diameter to 1,000 meters in the eruption, and boulders weighing as much as 14 tons were thrown into the air.

This boulder, ejected from Halema’uma’u during an explosion on 18 May 1924, was tossed 2,000 feet from the crater. Credit: U.S. Geological Survey; Hawaiian Volcano Observatory, courtesy of Bishop Museum

Until last week, attention on the latest eruption had focused on the ongoing flow of lava out of fissures in Kīlauea’s eastern flank, which has forced the evacuation of more than 1,000 residents from Hawaii Island’s Puna district and destroyed 37 structures to date. Last week’s warning shifted attention to the volcano’s summit crater, more than an hour’s drive away.

Because of the danger of steam-driven explosions, also known as phreatomagmatic eruptions, the U.S. National Park Service on Friday closed Hawai’i Volcanoes National Park, which includes the summit; the U.S. Federal Aviation Administration issued a flight restriction around the area; and only essential personnel remained at HVO near the edge of the crater.

These types of explosions have occurred at volcanoes elsewhere in the world. The 2014 eruption of Mount Ontake in Japan, which killed 63 people, was caused by pressurized steam. Although they can be deadly, such explosions are unusual for Kīlauea, Johnson said; a 1984 paper estimated that only about 1% of eruptions at Kīlauea over its history have been explosive. That’s largely because the magma’s low silica content and hot temperature make it so runny. “It’s unusual for pressures to be built up,” Johnson said. “Usually, gas is able to escape.”

Draining Magma

“In general, we understand [that] the more explosive phases occur when there’s a very low magma supply to the volcano,” she said.Still, Kīlauea seems to undergo cycles of explosive-style eruptions, said USGS volcanologist Wendy Stovall. In addition to the 1924 event, the volcano had an explosive eruption in 1790, she said. “In general, we understand [that] the more explosive phases occur when there’s a very low magma supply to the volcano,” she said.

Kīlauea’s surface swells as underground reservoirs fill with magma, and then subsides, or deflates, as reservoirs empty. The lava lake at Halema’uma’u began draining on 2 May, with the level dropping 2 meters per hour until the lake’s surface disappeared from sight on Friday, USGS scientists said.

Since then, using tiltmeters, GPS, and satellite radar interferometry, HVO volcanologists have observed steady subsidence of the volcano’s summit, an indication that the magma column is still receding. The amount of subsidence is likely a matter of millimeters, Johnson said—too small to be noticeable to the naked eye but able to be detected with volcanologists’ sensitive instruments.

An ash plume rose from the crater at Kīlauea’s summit on Friday, probably indicating a rockfall within the crater. Credit: U.S. Geological Survey

Plumes of gas, smoke, and steam billowing up from deep within the crater provide further clues to what’s happening underground, Johnson said. Brown or gray plumes indicate sulfur dioxide or ash. “What they’ve been seeing are some very white ones, which suggests steam,” she said. “That suggests there’s water coming into contact with the magma.”

Scientists have also observed a number of rockfalls from the steep sides of the now empty crater, activity that can contribute to plugging the vent, she added. Rocks falling from the exposed walls of the crater can also agitate the surface of the lava lake, as seen in a USGS video shown in this tweet from the agency. The video was taken on 7 May, 5 days after the lava level began to drop.

Scientific Opportunity

Even though scientists may be seeing conditions similar to those that preceded Kīlauea’s other explosions, they’re not on the same scale, said Tina Neal, the scientist in charge of HVO. “We have yet to see the kind of drawdown or subsidence or deformation at the summit that occurred in 1924,” she said. Moreover, any explosions would be localized around the summit, not blow-the-top-off catastrophic blasts like the one at Mount St. Helens in Washington State in 1980, USGS scientists said.

That’s because Kīlauea is a different type of volcano than Mount St. Helens. Kīlauea has runny lava and an open vent that has allowed an ongoing release of pressure. A relatively small mass of rock sits on top of Kīlauea’s magma, whereas Mount St. Helens’s much stickier lava had long trapped gases beneath a tremendous rock mass, explained Johnson. When a landslide suddenly removed some of the Washington State volcano’s overlying material, the pressurized gas was able to blow the rest away.

“This could be a completely new data set for us,” she said. “Why is the lava lake being drained away? And what is it that triggers this change in eruptive behavior?”Although Kīlauea’s 1924 explosion was observed by scientists, including the famous volcanologist Thomas Jagger, who described it in a paper, the current eruption could provide an opportunity to study the phenomenon with modern tools, Johnson noted.

“This could be a completely new data set for us,” Johnson said. “Why is the lava lake being drained away? And what is it that triggers this change in eruptive behavior? That [last question], I think, is a fundamental question that this sequence can possibly provide data for.”

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